U.S. Pat. No. 4,312,053 issued Jan. 19, 1982 to Lipsky for RANGE AND DEPTH DETECTION SYSTEM discloses a method and apparatus for computing the range to and depth of a submerged object that either emits or reflects pulses of acoustic energy. A detecting array and related data handling apparatus measures as a function of time, the apparent angles of the arrival of the acoustic energy associated with each such pulse.
U.S. Pat. No. 5,555,668 issued Sep. 17, 1996 to Brasseur for METHOD AND DEVICE FOR LOCATING BAIT AT DESIRED DEPTH WHILE FISHING and discloses a bait locating device for accurately positioning bait adjacent a particular fish to be caught. The depth of the device is determined by an appropriate length of sinker line which is dependent upon the length of an employed hook snell and determined distance of fish from the bottom.
U.S. Pat. No. 5,563,849 issued Oct. 8, 1996 to Hall et al. for ACOUSTIC TRACKING SYSTEM and discloses an acoustic tracking system designed to allow scientists and fishermen to follow dolphins and whales without harming or harassing them. The system utilizes flow shielded hydrophones mounted on the bottom of a vessel in two arrays. Acoustic signals of interest strike the hydrophone piezo-electric elements and are transmitted to a receiver-display which determines and displays both the bearing to the signal of interest and the relative range from the vessel to that signal.
U.S. Pat. No. 6,820,366 B1 issued Nov. 23, 2004 to McDermott for AUTOMATICALLY ADJUSTABLE FISHING LURE discloses a fishing lure that is automatically adjustable in order to maintain the lure at an optimum depth in a body of water. The lure contains within the lure itself, a central processing unit, a sender/receiver, and a depth varying mechanism. In response to signals from the sender/receiver, the control processing unit sends signals to control the depth varying mechanism in order to maintain the lure at a constant distance from either the bottom or the surface of the body of water.